VS: Do not apply any '/external:*' flag table mapping on VS < 16.10

Since commit 887e9df0 (VS: Update v142 CL flag table for VS 16.10,
2021-06-04) we map several `/external:*` flags to their corresponding
`.vcxproj` elements.  These elements were added to `cl.xml` in VS 16.10,
so filter them out in older VS versions.  Add a field to the json flag
table format to specify the minimum version of VS needed for a given
mapping.

Issue: #22308